PROJECT SUMMARY/ABSTRACT Septic shock with multiple organ failure is the leading cause of death in non-coronary intensive care units and remains a major health problem in the US. Endothelial cell (EC) dysfunction, manifested by increases in vascular permeability, is an important hallmark of septic shock and plays a critical role in the pathogenesis of multi-organ failure. Pericytes maintain endothelial barrier function and represent a potential therapeutic target in sepsis. However, the processes that govern pericyte viability and their role in barrier function in sepsis have not yet been fully elucidated. Pericytes are specialized cells embedded in the capillary basement membrane that wrap around endothelial cells of the microcirculation throughout the body and are thought to be important regulators of microcirculatory homeostasis. However, the role of pericytes in the endothelial dysfunction of sepsis is largely unknown. Our data demonstrated that pericytes are depleted in the mouse lung and kidney microvasculature during cecal ligation and puncture (CLP)-induced sepsis and pericyte depletion results in vascular leakage. The transcription factor friend leukemia virus integration 1 (Fli-1), is critical to pericyte dysfunction and viability in sepsis by mediating pericyte programmed cell death through pyroptosis. MiR-145 inhibits Fli-1 expression and is abundantly expressed in pericytes; however, during sepsis, miR-145 expression decreases with a concomitant increase in Fli-1 expression. This suggests that miR-145 may play an important role in pericyte viability and, therefore, be an important regulator of vascular permeability. A clearer understanding of how the miR-145/Fli-1 axis regulates pericyte viability in sepsis in a critical gap in knowledge that may lead to novel therapeutic approaches for sepsis. Furthermore, understanding the mechanisms by which pericytes support the vascular barrier integrity in sepsis may also have therapeutic implications. One potential mechanism whereby pericytes communicate with endothelial cells is through paracrine signaling via exosomes. Intriguingly, we have demonstrated that pericyte-derived exosomes but not fibroblast-derived exosomes improve survival in mice subjected to CLP-induced sepsis and that pericyte exosomes contain abundant miR-145. The long-term goal of our research is to identify novel treatment strategies to maintain endothelial barrier function in sepsis. The overall objective for this proposal is to identify the determinants of pericyte viability in sepsis and the mechanisms by which pericytes maintain endothelial barrier function. We hypothesize that pericyte viability regulated by the miR-145/Fli-1 axis is a critical determinant of sepsis outcomes through pericyte- mediated stabilization of endothelial permeability. Three specific aims address this hypothesis: Aim 1: Determine the mechanisms by which the miR-145/Fli-1 axis regulates pericyte function and viability in sepsis. Aim 2: Define the mechanisms by which pericytes and pericyte exosomes regulate EC function. Aim 3: Elucidate the therapeutic potential of pericyte-derived exosomes in CLP-induced sepsis.